An RP/E (Repair Party Electricity) marked to the front, with an M for messenger to the rear. Above the RP/E is B.G.B.C. which I think is ? ? Borough Council.
Underneath the RP/E & black paint is FAP, & underneath the M is another M, so has seen a bit of use.
No markings on the shell, the liner has an row of O's which I have seen before, & the liner being dated 1937 can only be HELMETS Ltd.